It is generally known that acoustic measurement can be used to determine properties of a wood product, such as a log, tree, board, or the like. These properties may include, for example, stiffness, strength, shrinkage, and other characteristics. In some embodiments, in which properties of a wood product are being ascertained, a stress wave is induced into the wood product. Next, a measurement is taken with respect to the time in which the stress wave travels from a first end to a second end of the wood product. From this time interval, a velocity of the stress wave can be determined via the equation:v=d/t
Where “v” is velocity of the stress wave; “d” is the distance traveled by the stress wave; and “t” is the time period of travel. This method of determining velocity is commonly referred to as a “time-of-flight” method. The velocity can, for example, be correlated to a modulus of elasticity (“MOE”) for the wood product, which is an indicator of the stiffness of the wood product. The velocity can also be correlated to warp potential for the wood product.
These types of measurements are most commonly taken on the outer wood, or mature wood, of a standing tree to assess stiffness and warp propensity of the lumber converted from a pre-harvest forest stand. Trees and stands demonstrating high values for stress wave velocity (“SWV”) generally will produce lumber that is stiff and stable, as well as less prone to warp.
Juvenile wood, or wood comprising approximately the first 10-15 growth rings has low stiffness, has a steep shrinkage gradient, and is more prone to warp than mature wood. As a result, the outer wood measurement of stress wave velocity will over-estimate the stiffness and underestimate the warp propensity of the recovered lumber which contains a large amount of juvenile wood. FIGS. 1 and 2 illustrate the growth rate effect on measurement of stress wave velocity versus actual stiffness of lumber derived from a log via a cant (i.e. the stress wave velocity estimation problems associated with logs and trees). In addition to the diverse site and genetic factors, plantation stands have different silvicultural prescriptions during their long rotation. Thus, although the diameter, or the age, or both, of pre-harvest stands may be the same, the growth ring patterns of the trees could be very different. Previous studies have taught mathematical corrections for both diameter and SWV for the evaluation of MOE of logs. However, these corrections do not compensate for the impact of a percentage of juvenile wood in a sample.
It is known that stiffness is the one of the most deficient properties for structure wood products, and straightness is one of the most important factors in a lumber buying decision for builders. Therefore, stiffness and warp propensity are important properties of trees and logs used to manufacture wood products. Stiffness and warp propensity varies significantly within and between forest stands, and this offers an opportunity to rank and sort the trees and stands for genetic improvement and for allocating a particular material to the appropriate manufacturer to optimize the value through the forest cycle. Visual characteristics such as size and morphology of crown, stem, and branches may offer some indications of wood properties; however, trees or stands with identical morphologies often have very different stiffness and warp propensity levels. Rapid, nondestructive methods have been applied to sort and rank internal wood properties such as stiffness and warp propensity of trees, logs, stems or forest stands. However, these methods often cannot predict or rank MOE or warp propensity sufficiently because they do not compensate for growth rate differences.
A need, therefore, exists for a method for adjusting property calculations based on stress wave velocity measurements to compensate for growth rate differences amongst timber-based raw material groups.